I'm building a racer but I don't know how to create an acceleration sound, I can make the car make a sound when going forward but I'm looking for a proper rev up/down so it gradually revs up as the car gets faster (and vice verser when slowing down).

Normally you would change the pitch of the sound file, but Construct 2 doesn't have a pitch setting, so try changing the playback rate of the audio file. Set faster playback for rev up, and slower for rev down.